The present invention relates to light guide sensors and, more particularly, an optical electric current sensor.
Prior Faraday effect electric current sensors use single-mode fibers. These fibers must maintain the linear character of the wave polarization, therefore resulting in the desirability to use an optical fiber which is non-birefringent or has very low birefringence.
An out-of-round fiber core, sometimes due to uneven stresses accumulated during cooling when the fiber was drawn, is characterized by the introduction of intrinsic birefringence in the optic fiber.
Further, the fiber, being wound around the electric current conductor imparts an additional birefringence source, known as curvature birefringence.
These two linear birefringences have the major disadvantages of masking and interfering with the sought-for Faraday effect. This effect appears in a non-birefringent medium and under the effect of a magnetic field line collinear with the direction of propagation of the light wave, through the rotation of the polarization vector. The magnetic field line can originate from the circular magnetic field produced by the electric current in the conductor.
A solution proposed by S. C. Rashleigh and R. Ulrich in the article "MAGNETO-OPTIC CURRENT SENSING WITH BIREFRINGENT FIBERS", published in Applied Physic Letters, Vol. 34, No. 11, pp. 768 to 770 (1979), is the elimination of the perturbations due to the linear birefringences. In other words, it is a case of eliminating the linearly birefringent character of the optical waveguide, or dominating it by means of a strong circular birefringence. The article shows the use of a twisted fiber. In such a fiber, the circular birefringence introduced makes it possible to decrease the influence of the linear birefringence of curvature and to use a fiber with a long length.
A second solution is proposed by L Li et al. in the article "CURRENT SENSORS USING HIGHLY BIREFRINGENT BOWTIE FIBERS", published in Electronics Letters (1986), Vol. 22, No. 21, pp. 1142-1144. L Li et al. show that the effects of the presence of the internal and external linear birefringence can be controlled by using an optical fiber whose elliptical birefringence is high. Such a light guide is obtained by twisting, during its drawing, a fiber with high linear birefringence.
In such systems, the value of the electric current is deduced from the measurement of the rotation of the polarization plane.
Other solutions based on the same principle are described, for example, in the publications of:
M. Kuribara et al. "CHARACTERISTICS OF TWISTED SINGLE-MODE OPTICAL FIBER FOR CURRENT SENSOR", Transl. Inst. Electron. and Commun. Engl. Jpn. Part C (Japan), Vol. J66C, No. 2., February 1973; PA1 A. M. Smith "POLARIZATION AND MAGNETOOPTIC PROPERTIES OF SINGLE-MODE OPTICAL FIBER", Applied Optics, Vol. 17, No. 1, January 1978, or again PA1 "OPTICAL FIBERS FOR CURRENT MEASUREMENT APPLICATIONS", Optics and Laser Technology, February 1980, by the same author.
Although the theory of coupled modes is known and was set out in 1973 by A. Yariv in IEEE Journal of Quantum Electronics (September 1973, pp. 919-933), or more recently by Scott C. Rashleigh in the Journal of Lightwave Technology (Vol. LT-1, No. 2, June 1983, pp. 312-331), and although mode coupling structures and their characteristics have been known and used for a long time in various fields (the U.S. Pat. No. 3,891,302 granted in June 1975 and the patent application GB 2,125,572A filed in August 1982), the state of the art shows that the Faraday effect electric current sensors of the prior art us a single mode optical fiber and are based on the elimination or domination of the intrinsic linear birefringence and birefringence of curvature of the said optical fiber.